The invention relates to a method for measuring properties of a moving web, the method comprising applying a stimulus to the web and measuring the effect of the web on the stimulus, the web being supported by a sheet at the measuring point.
The invention further relates to an apparatus for measuring properties of a moving web, the apparatus comprising at least one excitation element for applying a stimulus to the web and at least one detection element for detecting the radiation transformed by the web and at least one sheet for supporting the web at the measuring point.
Properties of a moving web are usually measured by means of a traversing measuring frame. For many measurements, for example color and basis weight measurements, the web needs to be unsupported in prior art solutions. For example in color measurements the means of measurement requires a specific reference material opposite the sensor.
When using reflection measurement on a supported web, the measurement contains both emissions reflected by the web and emissions which were transmitted through the web, reflected by the support sheet, and transmitted back through the web. Since the web is opaque only to certain emissions, the reflection from the support sheet might cause a significant and variable bias to the measurement.
In bulky measurement devices and especially in devices which traverse the web or in apparatuses containing plural devices deployed across the web, the path length over which the web is unsupported may be significant. Stresses on the web generally increase rapidly with the increase of speed of conveyance or processing. Thus the unsupported sections can limit the speed of processing the web, since there is a maximum stress the web can bear. Moreover, since properties of a web normally exhibit some variation, the maximum bearable stress may vary. This leads to a greater incidence of interruptions to production caused by web breaks in the unsupported sections. The measuring frame itself and its measuring carriages are also harmful in a fast machine since the gap between the upper and lower parts of the carriage is very small, and in order to avoid marks the sheet should not touch the carriages. An unsupported web may also flap, and if it then goes between the measuring carriages, it is highly probable that this will cause marks, or even holes, to the sheet.